1. Field of the Invention
This invention relates to a high strength non-heat refining free cutting steel having high strength as well as excellent machinability (free cutting property) and suitable for use as materials for components of various machine structures such as an automobile requiring the machinability at the time of manufacture and the high strength at the time of practical use.
2. Description of the Prior Art
Heretofore, it has been tried in various fields to apply non-heat refining steels which are not subjected to quench-and-temper treatment. The conventional non-heat refining steels have a hardness up to Rockwell C 25 in the majority cases, and it is relatively easy to machine them because they have been used mainly instead of quench-and-tempered medium carbon steels.
In addition, if it is necessary to further improve the machinability, the steels are added with free cutting elements such as S, Pb and the like in proper quantity. In this case, since the hardness of the steels is low and the notch sensitivity is not so high, therefore the deterioration of the strength is not brought on.
However, in case of the non-heat refining steels having a hardness exceeding Rockwell C 25, there is a problem in that the steels are apt to be deteriorated in the fatigue strength by containing the free cutting elements such as S, Pb and so on in order to improve the machinability of the steels.